In the course of driving heavy vehicles such as overland trucks and buses (which should be considered interchangeable for purposes of the description contained herein), it is common to be required to drive at relatively slow speeds, often for extended periods of time. Exemplary situations are driving in slow, backed up traffic and maneuvering about loading yards where high-speed travel is not possible. In modern heavy vehicles, it is common to find that such vehicles are equipped with an automatic mechanical transmission (AMT) or an automatic transmission. In either case, computer control strategies are utilized in the selection of gear engagements, as well as transition strategies between the different gear choices of the transmission.
Referring to the situations in which it is desired that the heavy vehicle move slowly but substantially constantly on course, operators have developed habits for engaging an appropriate low gear which carries the vehicle forward or backward under the power of the idling engine. Depending upon the desired speed and the heavy vehicle load, among other factors, different low gears are selectable.
The low gears available for selection, however, are limited by the torque that can be developed in each gear by the engine operating at the preset idle speed, and the range of gears available for use at any particular time will be determined by conditions of the vehicle, as well as conditions of the environment within which the vehicle is operating. The two primary conditions upon which the range of available gears is dependent is the mass of the vehicle (including any load) and ground inclination, and each of these two aspects bear on the vehicle's resistance to travel, as does wind/air resistance. Dependent at least in part on each of the two characteristics of vehicle mass and ground inclination, the highest gear of the transmission can be determined at which the idling engine can maintain a substantially constant speed of the vehicle without losing speed because of insufficient torque capability. Heretofore, operators have been left to draw on their experience for selecting an initial gear for establishing such engine idle-driving mode, with adjustments being made up or down in order to engage the gear which produces the desired travel speed, and which is also capable of maintaining that speed using the torque developed at the preset idle speed of the engine, for example, 650 revolutions per minute, give or take a few hundred revolutions, depending on the particular engine.
It is appreciated that if presently existing conditions are known which bear upon the highest gear selection at which the idling engine can maintain a constant vehicle speed, that gear can be determined, engaged and utilized for powering travel of the vehicle. Often times, however, the highest possible gear ratio carries the vehicle in the engine idle-driving mode at a groundspeed greater than desired. For instance, the traffic flow within which the heavy truck is operating maybe slower than this maximum speed which the idling engine can maintain under existing conditions. Heretofore, as described above, selection of the proper gear which permits the engine to operate at idle and produce the desired speed of the vehicle was performed by the operator himself based on past experience and trial-and-error with respect to selection within a typical low range of gears.
This type of trial-and-error, hunt-and-peck gear selection by the operator obviously has drawbacks; among others, if the truck is operating under slow speed conditions, the driver can become unnecessarily fatigued by the gear selection process. Still further, operating economy can suffer not only because of inefficiencies associated with constant gear changing and adjustments, but also if the optimal gear is not selected which can use the preset idle speed of the engine for maintaining the desired vehicle speed. Therefore, the need has been recognized for a drivetrain control system in which such gear selections are made on at least a semi-automated basis with only minimal or no direct selection input from the operator.
In at least one exemplary embodiment, the present invention takes the form of a method for exiting or leaving an engine-idle driving mode in a heavy land vehicle that has an automatic transmission. In the present context, the terminology “automatic transmission” is used to identify transmission configurations that are fully automatic (no accommodation for manual gear selection), as well as those often referred to as semi-automatic because gear selections can be operator-designated, but the transmission also has automatic gear changing features. As explained in greater detail above, the engine-idle driving mode is established in a vehicle when the engine is running substantially constantly at idle speed and a gear is engaged propelling the vehicle at a corresponding substantially constant, but relatively slow vehicle speed such as in slow-moving, heavy traffic. When engine-idle driving mode is no longer needed, such as when the driver has cleared the slow moving driving pattern, their action is to depress the accelerator to increase the vehicle's speed. According to the invention, this action is recognized (assessed) as a driver request to exit or leave engine-idle driving mode and accelerate from the prevailing substantially constant, relatively slow vehicle speed. It will be appreciated by those skilled in the art that when the vehicle is traveling at such a relatively slow rate of speed, the engine may in fact not be imparting a great deal of power to the drive wheels so engine torque production may be minimal at the time. The torque capacity of the engine, however, is much greater and is generally known, for instance, according to the engine's torque curve.
Because of the driver's desire for an increase in vehicle speed (acceleration), more torque from the engine is needed. The amount of torque needed, however, will be determined at least in part based on the increased degree by which the accelerator has been depressed. In another aspect, the increase in delivered engine torque is based on the vehicle's resistance to travel. For instance, if the vehicle is on an incline, a greater amount of torque will be needed for the requested speed change than if rolling downhill. In a related manner, the weight (mass) of the vehicle also affects the amount of engine torque to change the speed of the vehicle. Additional situations where a greater amount of torque is needed include but are not limited to situations where an additional load is placed on the engine by a power take off unit or the vehicle is on soil that slows movement. The vehicle's resistance to travel and acceleration, however, can be at least generally quantified at any given time. The quantification can take place using either individual sensors for the mass, wind resistance, and power take off load, or using the current engine torque and vehicle acceleration to determine these resistances. Using this quantification, the present invention assesses whether the engine torque requirement for delivering the requested acceleration to the increased speed can be developed from the engine while maintaining the automatic transmission in the same gear presently engaged. Assuming that it is assessed that the engine can provide the required torque without downshifting the automatic transmission, then the extra power is delivered and the requested acceleration affected. Among other benefits, this positively affects drivability of the vehicle by smoothing take-off when the driver exits engine idle-driving mode. It also saves fuel over what would be consumed in normal driving mode due to downshifting that would occur if the same driver request for increased vehicle speed were made while driving at the same relatively slow vehicle speed, but not in engine-idle driving mode.
Exemplarily, the driver request to accelerate the vehicle is assessed from the detection of a change in depression of an accelerator of the vehicle, and more specifically, an increase in depression. When leaving the engine idle-driving mode, the increase in depression of the accelerator normally begins from a non-depressed position; i.e., a foot-off-the-gas situation.
As shown in FIG. 1, another aspect of the invention is that while in the engine-idle driving mode (block 10) and a driver requests acceleration (block 20), it is assessed whether available engine torque in the current gear is sufficient to maintain the substantially constant, relatively slow vehicle speed (block 30) during times when vehicle travel resistance is fluctuating. A prime example of such a fluctuation in vehicular resistance to travel is the influence of road inclination. For instance, as a vehicle travels along an incline-changing road such as rolling hills and valleys, the resistance to forward travel commensurately changes thereby constituting fluctuations in travel resistance. Preferably, this assessment is conducted substantially continuously. The requested acceleration is delivered by increasing the engine's torque output and without downshifting the automatic transmission (block 40).
As mentioned above, one important variable that can influence a vehicle's resistance to travel is its mass or weight, and the effect of the vehicle's weight is accentuated when road inclination is simultaneously considered. Another influence is air resistance, and particularly wind resistance which has a fluctuating character based primarily on strength and direction.
One aspect of the engine idle-driving mode is that the automatic transmission is downshifted at least one gear when it is assessed that maximum available engine torque in the current gear is insufficient to maintain the substantially constant, relatively slow vehicle speed when vehicle travel resistance is considered.